The present invention relates to a reactive power compensation apparatus and, more particularly, to an effective reactive power compensation apparatus which suppresses voltage fluctuation and unbalance in an AC power supply system so as to stabilize the power supply system.
In recent years, a load requiring a single-phase power such as an AC-powered electric train is coupled to a power supply system, and this causes a voltage unbalance of the power supply system due to the influences of an unbalanced current and an impedance of the power supply system. Voltage fluctuation upon a change in reactive power is also a problem. For this reason, the power supply system is equipped with a reactive power compensation apparatus to compensate for the reactive power of the power supply system and suppress the voltage fluctuation. Furthermore, the unbalanced current of the power supply system is compensated for to remove an unbalanced component of the voltage.
A power supply system equipped with a reactive power compensation apparatus is described in detail in an article entitled "Simulator Test for System Stabilization by Static Var Compensator (SVC) using Digital Controller" announced in Denki Gakkai, Denryoku Gijutsu Kenkyukai in July, 1985. The basic arrangement of this system is as shown in FIG. 1.
Referring to FIG. 1, reference numerals 10 and 11 denote power supply buses of branch lines coupled to the load such as an AC-powered electric train; 4, an AC power supply system; and 100, a reactive power compensation apparatus, which comprises reactor part 300 and phase-advanced capacitor part 200. Reactor part 300 comprises reactors 302U to 302W, cross-coupled thyristors 301U to 301W connected in series therewith, potential transformer 70, and control circuit 350 for thyristors 301U to 301W. Firing angles of thyristors 301U to 301W are adjusted by circuit 350 in accordance with a voltage detection value of power supply bus 6, thereby controlling current IR of reactor part 300. Note that reference numeral 3 denotes an impedance of a triple-phase AC power supply system; and 1, a triple-phase AC power supply system for trunk lines.
A power capacity (phase-delayed capacity) of reactor part 300 of apparatus 100 is normally set to be twice a power capacity (phase-advanced capacity) of phase-advanced capacitor part 200. When reactor current IR is changed from zero to maximum, power Q generated from apparatus 100 can be smoothly changed from an advanced-phase to a delayed-phase, as shown in FIG. 2.
In the power supply system with the above arrangement, if a reactive current (or reactive power) of bus 6 is changed, a voltage of bus 6 fluctuates upon a change in current (power) and impedance 3. When an unbalanced current generated by a single-phase load such as an AC-powered electric train flows through bus 6, the voltage of bus 6 is unbalanced due to the influence of the unbalanced current and impedance 3. The reactive power compensation apparatus is adopted to suppress and compensate for voltage fluctuation and unbalance. However, the performance of the apparatus is determined by the detection precision of the voltage fluctuation and unbalance.
The conventional reactive power compensation apparatus has the following drawbacks. More specifically, the voltage fluctuation of AC bus 6 includes a component caused by a change in positive-phase current (positive-phase voltage fluctuation) and an unbalanced component of a voltage caused by a negative-phase current (negative-phase voltage fluctuation). In the conventional voltage detection technique, the positive-and negative-phase voltage fluctuation cannot be distinctly separated. The voltage fluctuation of bus 6 is regarded simply as a fluctuation component arbitrarily including positive- and negative-phase voltage fluctuations, and the reactive power compensation apparatus is controlled based on this. For this reason, in the conventional reactive power compensation apparatus, an object to be compensated cannot be theoretically identified, that is, it cannot be identified whether positive-phase voltage fluctuation (in particular, fluctuation caused by a positive-phase reactive current) or negative-phase voltage fluctuation, i.e., an unbalanced component of a voltage, is to be controlled. Therefore, high-precision control cannot be achieved.
A strong demand has arisen for the improvement of power quality in an AC power system and hence for a reactive power compensation apparatus for stabilizing a power system, capable of higher-precision control. In order to meet such a demand, development of a reactive power compensation apparatus comprising a high-precision voltage detection technique (positive and negative-phase voltage fluctuation detection) based on a novel control principle has received a great deal of attention.